1. Field of the Invention
The present invention relates to a scanning image forming lens and an optical scanning apparatus.
2. Description of the Related Art
An optical scanning apparatus for optically scanning a surface to be scanned at a constant velocity by deflecting a luminous flux formed as a linear image, in a direction corresponding to a main scanning direction, via an optical deflector having a deflecting reflective plane located in the vicinity of the linear image, so as to transmit the deflected luminous flux to a scanning image forming lens which collects the deflected luminous flux on the surface to be scanned as a light spot, is well known in an image forming apparatus such as an optical printer, a digital copier, and the like.
The above-mentioned main scanning direction refers to the direction corresponding to a scanning direction on an optical path extending from a light source to a surface to be scanned. A sub-scanning direction is a direction corresponding to a scanning direction along the optical path which is substantially perpendicular to the main scanning direction. The plane swept by the principal ray of a luminous flux which is optimally deflected by an optical deflector is referred to as the deflected plane in this specification.
The above-mentioned scanning image forming lens is designed to achieve a well known and desired conjugating function by which a geometrical-optically conjugate relationship between the image forming position of the linear image and the surface to be scanned in the direction corresponding to the sub scanning direction is achieved. Also, the above-mentioned scanning image forming lens is also designed to achieve a constant velocity function for having the optical scan performed at a constant velocity. The above-mentioned conjugating function is desired for compensating or correcting for a tilt or positional error of the deflecting reflective plane in the optical deflector which causes errors in scanning accuracy.
In addition to the condition that the above-mentioned conjugating function and constant velocity function of the scanning image forming lens are achieved, the curvature of field must also be accurately corrected and compensated for in the main and sub scanning directions in order to achieve a good optical scan. Unless the curvature of field is sufficiently corrected in the main and sub scanning directions, the light spot diameter changes according to the image height of the light spot, resulting in a significant reduction in the resolution of the image to be written, and consequently causing deterioration of the image quality of the written image.
A scanning image forming lens with the radius of curvature in the sub scanning cross-section (a flat cross-section which is substantially perpendicular to the direction corresponding to the main scanning direction in the vicinity of the lens surface) which has been changed according to positions in the direction corresponding to the main scanning direction in at least one lens surface of the scanning image forming lens for compensating the curvature of field in the sub scanning direction is disclosed, for example, in Japanese Unexamined Patent Publication No. 6-230308.
The radius of curvature in the sub scanning cross-section is changed so as to increase continuously in accordance with increasing distance of the position from the optical axis in the direction corresponding to the main scanning direction in the scanning image forming lens disclosed in JP 6-230308. Although the curvature of field in the sub scanning direction is corrected as described above, the above-mentioned radius of curvature drastically differs at the deflection angle of 0 (corresponding to the optical axis portion of the effective main scanning region) and at the maximum deflection angle (corresponding to the end portion of the effective main scanning region). In addition, the lens having the surface in which the curvature of field in the sub scanning direction is corrected, is not symmetrical or "asymmetrical" relative to an optical axis of the scanning image forming lens which is axis a in the Figures of JP 6-230308.
As a result, when a lens with the surface shape as disclosed in JP 6-230308 is assembled in a scanning apparatus and if an assembling error such as decentering, shifting, or the like, occurs, the curvature of field in the sub scanning direction is significantly deteriorated. Therefore, the above-mentioned scanning image forming lens has a problem in that positional accuracy of mounting and fixing the lens in an optical scanning apparatus must be very high and has very strict positional tolerance which makes the assembling process extremely difficult, time consuming and expensive and still results in a high probability of significant deterioration of curvature of field caused by assembling errors.
A scanning image forming lens such as that described above can be produced by plastic molding. However, sometimes it is difficult to obtain a lens with a desired shape due to sinking of a part of the surface of the lens (generally referred to sinking or waving) during the molding process. Furthermore, the optical characteristics of a lens produced by plastic molding can be easily changed by the influence of the temperature and humidity. Moreover, since a rotating polygon mirror commonly used as an optical deflector does not have the rotation axis of the deflecting reflective plane in the deflecting reflective plane, a "sag" condition where the positional relationship between the linear image formed in the vicinity of the deflecting reflective plane and the deflecting reflective plane changes according to the rotation of the rotating polygon mirror, may occur.